This invention relates to a method and apparatus for depositing material on a substrate.
The invention is particularly, but not exclusively, applicable to the deposition of an encapsulating material on a substrate already bearing sensitive material. For example, a semiconducting substrate bearing an array of organic light emitting diodes (OLEDs) may have a top electrode comprising a layer of low work function material such as calcium. Calcium is extremely reactive with oxygen and water and it is therefore required to coat the calcium layer with an encapsulating layer, for example of aluminum oxide or silicon dioxide, in order to prevent the ingress of oxygen and water.
One method of deposition of the encapsulating material which has been suggested is electron beam evaporation. This involves causing a hot filament to emit electrons, which impact the surface of the encapsulating material contained in a crucible. Under a high vacuum the electrons cause the encapsulating material to be evaporated and transferred to the substrate.
A major disadvantage of electron beam evaporation is the presence of secondary electrons which are emitted from the encapsulating material. These secondary electrons would damage polymeric layers of an OLED such as to render useless any OLED device sought to be fabricated.
Magnetic fields have been suggested as a means of confining the secondary electrons. However, magnetic fields are ineffective when the surface of the encapsulating material in the crucible is rough or when the secondary electrons have a wide range of energies and/or directions.